1. Field of the Invention
The present invention relates to expandable intraluminal vascular grafts, generally referred to as stents. More particularly, the invention pertains to stents that are simultaneously biodegradable and capable of releasing therapeutic drugs.
2. Description of Related Art
Stents are implanted within various lumens vessels of the body in order to maintain the patency of such vessels. A variety of delivery systems have been devised that facilitate the placement and deployment of stents. The stent is initially manipulated while in its contracted state, wherein its reduced diameter more readily allows it to be introduced into the lumen and maneuvered into place. Once in place, it is enlarged to a diameter either greater than or equal to the diameter of the lumen so as to allow the free flow of fluid therethrough.
A system especially adapted for coronary applications employs a stent design that incorporates a combination of interacting elements which serve to automatically lock the stent into its enlarged configuration upon expansion. The stent is moved into position along a previously placed guidewire. Inflation of a balloon about which the stent is held causes the stent to expand and lock. Subsequent deflation of the balloon and withdrawal of the catheter and guide wire leaves the stent in place. Elements extending from the stent's surface engage the vessel walls to positively maintain the stent in position within the lumen.
Stents have heretofore typically been formed of non-toxic, substantially biocompatible metals such as stainless steel, tantalum, or gold. It has however been found that within typically about seven to twenty-one days the endothelial layer of the artery or vessel grows into and throughout the walls of the stent, at which point the stent's utility is substantially diminished and its continued presence may cause any of a variety of problems or complications. It has therefore been proposed to form stents of biodegradable or bioabsorable materials that are completely resorbed by the body within a period of time.
Continued pharmacological treatment of the vessel or condition that made the implantation of the stent necessary is often required or most desirable. Such treatment is typically most effective when administered locally and as a result, it has been suggested to rely on the stent for the delivery of drugs. Materials are known that are capable of absorbing certain drugs and then releasing them at a substantially predictable rate for a predeterminable period of time when subjected to a particular environment. By forming the stent of such drug impregnated material or by otherwise associating such material with the stent, the stent can achieve the dual purpose of maintaining patency while dispensing drugs.
The prior art has been unable to provide a stent that is simultaneously completely resorbable, that possesses the physical properties necessary to facilitate its implantation and to perform its primary function of maintaining the patency of the vessel for a period of time and that gradually releases a drug prior to its resorption. Previous attempts to impart the necessary physical properties to drug-releasing materials or to impart such properties thereto without compromising the drug-releasing properties or the efficacy of the drugs absorbed therein have been unsuccessful.